Add a function to globally set volume (aka AL_GAIN)

simgear/sound/soundmgr_openal.hxx

@@ -174,6 +174,16 @@ public:
      */
     bool stop( const string& refname );
 
+    /**
+     * set overall volume for the application.
+     * @param vol 1.0 is default, must be greater than 0
+     */
+    inline void set_volume( const ALfloat vol ) {
+        if ( vol > 0.0 ) {
+            alListenerf( AL_GAIN, vol );
+        }
+    }
+
     /**
      * set the position of the listener (in opengl coordinates)
      */